You have intermittent problems connecting via IMAP. Your mail client occasionally says it's offline, then comes back online a few minutes later.
You will probably see this in /var/log/maillog: mail dovecot: master: Warning: service(imap-login): process_limit (100) reached, client connections are being dropped
Problem Resolution
The first thing to check is normally that your max connections and max connections per IP settings in the IMAP server are increased. If you've already increased them to a much higher value than your expected IMAP connectivity volume, and you're still seeing a problem, it probably lies in Dovecot's max login daemon connections.
Created a file as follows:
/etc/dovecot/conf.d/42-service-imap-login.conf
Inside that file, put:
service imap-login {
service_count = 1
process_limit = 500
}
Then restart Dovecot to have it pick up the new settings:
/etc/init.d/dovecot restart
Source
